1. Field of the Invention
The present invention relates to double containment pipe fittings and their assembly.
2. Description of the Prior Art
The general concept of providing a double or dual containment pipe system wherein an inner carrier pipe is concentrically located within an outer containment pipe to deliver dangerous or hazardous fluids is well known and an accepted commercial practice. Historical applications for such systems have been found in the nuclear, gas petroleum production and refining and chemical processing industries. The inner pipe is used to transport the hazardous or toxic fluid while the outer pipe is present to confine any leaks. Thus, it is also known to provide the annulus between the concentric pipes with various types of detectors and/or drainage apparatus to handle leakage. Examples of double containment pipe assemblies are shown in U.S. Pat. Nos. 4,786,088; 4,886,305 and 4,930,544.
With the advent of stricter governmental regulation concerning the piping of petroleum products and hazardous chemicals, the anticipated increased use of various types of pipes in double containment applications is a certainty. As such, the structural design of these fittings and method of installing and repair of double containment pipes containing such fittings is necessary.
The present invention provides novel double containment fittings for forming a double containment pipe joint. The products that have been developed to date have not completely addressed some of the unique problems that arise when a pipe is placed within another pipe.
One such problem is the ease of maintaining, modifying or repairing such systems. To date, all above ground pressure systems have been installed without the ability to assemble or disassemble the piping or components in modular sections. Therefore, if a repair is to be made, the original system manufacturer/contractor must be called in to facilitate the repair. In many designs, a repair of a certain section would not be feasible or even possible due to the location of the system with respect to adjacent equipment or building parts. What would be very helpful to most facility owners is the ability to have a modular system that is capable of being readily disassembled. A system capable of being readily disassembled would give the facility engineer the ability to have the section requiring modification or repair sent to a shop type environment readily suitable and equipped to facilitate the change or repair.
Another problem is the ability to provide a means of internally anchoring the inner piping of a double containment piping system. In ordinary single wall, non-double containment piping, it is often required to provide a means of anchoring the piping at a fixed point. There are several reasons why this is often necessary. One such reason is to control or limit vibration that might occur during the operation of the piping system. Another reason is to control and direct thermal expansion and contraction of the piping system. By forcing thermal expansion/ contraction away from a point of anchorage, several ordinary layout means can then be used to "compensate" or "alleviate" the build-up of thermal stresses. This is ordinarily done by adding some means of flexibility and added degrees of movement into the piping system between points of anchorage. Also, anchors can sometimes be placed in sequence without flexible means placed between them. Thermal expansion can then be accommodated by allowing a controlled amount of displacement strains to occur. This is possible if the total of predicted stresses that result do not exceed the allowable for the material, taking into account any cyclic effects.
Thus, the use of anchoring devices is a key element to a successful pressure piping design in both metal and nonmetal systems. In double containment piping, sometimes the inner piping is subjected to a greater amount of thermal expansion or contraction than the outer piping. Sometimes, however, it is the outer piping that can experience the greater overall change in dimension. What would be highly beneficial would be to have a component which would allow both pipes to be anchored to each other, yet still maintain an area of "zone" of containment around the entire inner pipe. Such a component would also be highly useful if it were to be able to be added into a system having any possible combination of materials (e.g., metal inside of plastic), as well.
Another such problem has to do with the ability of a system to withstand the effects of inner and outer piping and components that are subjected to different amounts of thermal expansion and contraction. It is the norm, rather than the exception, that the inner and outer pipes of a pressure rated double containment piping system are subjected to different amounts of thermal expansion. This situation may arise in several different ways. The most common way involves the situation whereby a hot fluid is transported through the inner pipe. Under this circumstance, the external environment (external that is to the outside diameter of the secondary containment piping) is normally at a lower temperature than the hot fluid. Since there is either an insulating dead air space between the two pipes, or other insulating material, the inner piping temperature becomes close to that of the fluid, while the outside piping remains closer to that of the external ambient environment. Therefore, the materials normally grow to different lengths due to their being at different temperatures. When this does occur, there are thermal strains that are imposed on interconnecting parts and on parts such as internal anchors and termination fittings that create a contact point between the inner and outer piping. The most obvious place where there is a problem potential is at the interconnecting points. However, any place where loads can be transmitted back and forth between the two pipe systems can result in a problem. With the exception of the development described in this application, all restraint couplings (internal anchors and termination fittings) designed for this situation are constructed of a singular material, with such a design that the residual stresses and subsequent strain on the materials can lead to a failure of such parts. These existing parts then become the point in the overall system at which failure is most likely to occur. This is compounded by the fact that the components are truly single containment at that point, without having a containment area to prevent the hazardous fluids from reaching the environment. Therefore, exactly where it would be the least desirable location for failure to occur is the most likely place for failure to occur. Additionally, a double containment piping system that is constructed of an interconnecting part as described above, actually increases the chance that failure to the external environment can occur as compared to a singular containment piping system designed with proper treatment of the thermal expansion of its components.
Another problem with existing systems has to do with the ability to combine different inner and outer pipe materials in an efficient manner. Different materials can mean materials that are of a different class, (such as metallic-thermoplastic, metallic-reinforced thermosetting plastic, or thermoplastic-reinforced thermosetting plastic), or materials that are within the same class but constitute a different material, (such as within the thermoplastic family, combining a fluoropolymer within a polyolefin, or a polyolefin within another polyolefin). The reason that it is desirable to combine materials typically has to do with economics. It is desirable in many situations to combine an expensive material that is capable of handling a chemical on a full time basis within a less expensive material capable of withstanding the corrosive effects of a chemical for a limited period of time. Another major economic reason has to do with the use of a material for the outside piping capable of withstanding the corrosive effects of atmospheric conditions, thus eliminating the need for expensive coatings, cathodic protection, etc. A typical example of this would be in combining a metallic material within a nonmetallic outside material for the reasons just described. Yet a third reason has to do with structural concerns, such as where a non-metallic material is housed within a metallic outer jacket, for purposes of protecting a pipe that is buried to shallow depths from possible large "live loads" due to vehicular traffic on the ground surface.